How to reproduce:
1. Download attached project files
2. Open "HorizontalGroupBug" Scene
3. In Hierarchy Window go to Canvas -> Panel -> HorizontalGroup
4. Right click on HorizontalGroup and add an empty GameObject child (In the scene view note that child position is correct)
5. Select the new GameObject and change its Width from 100 to 200 (In the scene view note that the object is no longer correctly positioned in the HorizontalGroup)

Expected result: Modifying items in LayoutGroups should display properly in Scene View same as what the item will actually be in the game
Actual result: When modifying items in a HorizontalLayoutGroup or VerticalLayoutGroup, the item displayed in the editor is not a proper representation of what the item will actually be in the game

Reproduced with: 5.5.4p3, 5.6.3p2, 2017.1.0p5, 2017.2.0b9, 2017.3.0a5

Workaround: Go into Play Mode and once play has begun, change back to the Scene tab and note that the GameObject has moved and is now in the correct position.